N23 and N24, selected from hundreds of ad-3 mutants, have been used as testers for the spot, plate, and suspension tests in Neurospora crassa. These two testers are highly sensitive to mutagens and are revertible by specific groups of chemicals. N23 can be reverted from adenine-dependence to adenine-independence by agents which cause base-pair substitutions, whereas N24 can be reverted by frameshift mutagens. Spot, plate, and suspension tests using testers N23 and N24 are satisfactory substitutes for the ad-3 forward mutation system for screening the mutagenic activity of environmental agents and chemical carcinogens in N. crassa. Further studies will be conducted to validate these test systems and to determine whether the sensitivities of these testers to chemicals can be enhanced by incorporation of UV sensitivity and Osmotic markers into these tester strains.